Known heat exchangers, of the kind used in cooling and heating systems for buildings, generally comprise a reservoir filled with a phase-change medium through which a plurality of parallel heat exchange circuit pass. The phase-change medium is usually aqueous-based and changes between water and ice in order to store and release coldness. Typically the tubes which comprise the heat exchanger element are disposed in coiled formations extending between an inlet manifold and an exhaust manifold.
A known heat exchanger element includes vertically disposed spacer elements which maintain the relative positions of the coils one above the other. The tubes are spaced closely together and their forward flow portions are interspersed with their return flow portions in order to minimise the temperature gradient throughout the phase-change medium. In this way ice is intended to form simultaneously throughout the phase-change medium and displace water upwards, thus preventing any build-up of pressure against the walls of the reservoir. However, in practice reservoirs are often constructed from flexible materials in order to accommodate some degree of expansion.
Heat exchange elements have customarily been made by winding plastics tubing into coils. This is a slow and cumbersome operation and it is almost impossible to wind coils with even spacing and without crushing or kinking the tubing. Uneven spacing leads to the appearance of temperature gradients in the phase-change medium, and kinking causes unacceptable pressure drops through the coils. In addition, the plastic tubing is taken from spools, each of which may hold, say, a 100 meters of tubing, and the tubing has a memory set into it which causes a tendency for it to resume the coiled shape in which it was wound on the spool. This tendency makes it even more difficult to wind the tubing into the coils required for the heat exchanger element, and also makes it very difficult to maintain the tubing coiled in the required configuration.
A further problem encountered in the manufacture of heat exchangers is that the elements are required to be rigidly restrained within the reservoirs in order to prevent movement resulting from their buoyancy. The restraint has to be sufficiently strong to survive the commissioning of the heat exchangers, when water is often pumped into the reservoirs without close supervision.
There are many other problems associated with known heat exchangers and heat exchanger elements and their manufacture. For instance it has proven difficult to pressure test the parts of the assembly during construction which has led to the need for expensive pre-commissioning testing.